Significant industries revolve around the manufacture, sale and use of ski boats. For terminology purposes of this application, we will use the term “ski boat” (occasionally “sport ski boat”) to refer to any watercraft that falls within the common understanding of a ski boat, a sport ski boat (also known as “sport/ski” or “sport-ski” boats), a tow boat, or any comparable watercraft such as are designed and used for towing recreational or competition water skiers, barefooters, kites, wakeboarders, or tubers, irrespective of whether a particular boat is ever actually used for such purposes, and even though such boats may instead be used for other purposes such as fishing, cruising, patrolling, transport or the like.
Most inboard ski boats have non-steerable propellers that use a single rudder behind each propeller to control steering. Ski boats having a solitary non-steerable propeller, including fixed pitch and controllable pitch types, have the longitudinal centerline of their propeller shaft fixed in alignment with the longitudinal centerline of the watercraft. In a typical watercraft of this type, the propeller shaft is attached to the inboard motor; the shaft extends through the hull, is braced by a strut on the underside of the hull, and terminates at the propeller. Other inboard motor watercraft include more complicated configurations where a transmission, gearbox or other linkage connects the engine's drive shaft to the propeller shaft, such as with a “V-drive” propulsion system. For purposes of this patent application, embodiments tend to be described in terms of the simpler embodiments, such as where the propeller shaft is the same as the engine shaft, but description with reference to a single or simple structure should be understood to encompass more combined or complicated structures that can be substituted for the single or simple structure.
Regardless of such particulars, most inboard motor watercraft have a rudder positioned aft of each propeller, along the extended centerline of the propeller shaft. The aft rudders are typically controlled mechanically with a helm, like a steering wheel, that is mounted on the deck of the watercraft. Control linkage between the helm and rudders is often achieved with control cables or other mechanical linkage, although “drive by wire” electronic controls are also well known as substitutes for mechanical linkages, particularly on larger watercraft.
At medium to high hull speeds, water flow past the aft rudder(s) is sufficient to allow for responsive handling by the operator. However, at slow hull speeds and at low propeller thrust, there is little water flow past the aft rudder, and the steering system is less effective. With slow water speed past the rudder, such as is typically encountered when docking a watercraft, laminar flow on both surfaces of the rudder is tentative at best, and resulting steering forces (i.e., yaw moments of inertia) are very limited, as is the operator's ability to steer the watercraft with the rudder.
Comparable or worse challenges also arise when a watercraft is moving astern. While the aft rudder is effectively upstream of the propeller when moving astern, the water flow across the aft rudder can be even more reduced because the propeller wash is directed away from the aft rudder. As a result, the aft rudder provides reduced directional control. This reduced control makes it more difficult to successfully maneuver the watercraft, especially in a crowded area or near a dock or loading ramp.
Many other problems, obstacles, limitations and challenges of the prior art will be evident to those skilled in the art.